NLMIXED in R

NLMIXED Compare Models

try.nlm2<-nlme(y~U+D*exp(-(Kd*x)),
               data=Grass_compl,
               fixed=U+D+Kd~1, 
               random=U+D+Kd~1|Trial/Cow,
               start=c(U=20,D=50.88,Kd=0.02698),
               method="ML", 
               control=nlmeControl(maxIter=1000, msMaxIter = 100))

try.nlm3<-nlme(y~U+D*exp(-(Kd*x)),
               data=Grass_compl,
               fixed=U+D+Kd~1, 
               random=U+D+Kd~1|Trial/Cow/Replicate,
               start=c(U=20,D=50.88,Kd=0.02698),
               method="ML", 
               control=nlmeControl(maxIter=1000, msMaxIter = 100)) # Can't run, singular precision matrices 

try.nlm4<-nlme(y~U+D*exp(-(Kd*x)),
               data=Grass_compl,
               fixed=U+D+Kd~1, 
               random=U+D+Kd~1|id,
               start=c(U=20,D=50.88,Kd=0.02698),
               method="ML", 
               control=nlmeControl(maxIter=1000, msMaxIter = 100)) 

try.nlm5<-nlme(y~U+D*exp(-(Kd*x)),
               data=Grass_compl,
               fixed=U+D+Kd~1, 
               random=U+D+Kd~1|Cow,
               start=c(U=20,D=50.88,Kd=0.02698),
               method="ML", 
               control=nlmeControl(maxIter=1000, msMaxIter = 100))
               
nlm<-nlme(y~U+D*exp(-(Kd*x)),
          data=Grass_new,
          fixed=U+D+Kd~1, 
          random=U+D+Kd~1,
          start=c(U=20,D=50.88,Kd=0.02698),
          method="ML", 
          control=nlmeControl(maxIter=1000000, msMaxIter = 1000))

nlm_auto<-nlme(y~U+D*exp(-(Kd*x)),
          data=Grass_new,
          fixed=U+D+Kd~1, 
          random=U+D+Kd~1,
          start=c(U=20,D=50.88,Kd=0.02698),
          correlation = corARMA(c(-0.4,-0.2,0.3), p=0, q=3, form=~1|id),
          method="ML", 
          control=nlmeControl(maxIter=1000000, msMaxIter = 1000))                          
               
# Compare Nested Models 
anova(try.nlm1, try.nlm2) 
anova(try.nlm1,try.nlm5,try.nlm4)